ABSTRACT Dose adjustment of renally cleared drugs in patients with chronic kidney diseases (CKD) is currently based on serum creatinine concentration, a biomarker of glomerular filtration (GFR). Despite dose reduction, adverse drug reactions remain extremely prevalent in CKD patients. Often, observed changes in drug exposure do not follow predictions based on the decline in creatinine clearance (CLcr), as exemplified by penciclovir (PEN) and tenofovir (TEN). We assert that the primary cause of suboptimal drug dosing in patients with CKD is the failure of estimated CLcr to accurately reflect the functional decline of renal tubule secretory function in CKD patients, and that drugs cleared primaily by tubular secretion (vs filtration), are subject to further compromise in clearance due to inhibition of secretion by accumulating uremic solutes. Renal tubular secretion requires coordinated uptake transport at the basolateral membrane and efflux transport at the apical membrane of the proximal tubular epithelium. In vitro studies have established that accumulating uremic solutes such as hippuric acid (HA), indoxyl sulfate (IS), p-cresol sulfate (pCS), and trimethylamine N-oxide (TMAO), inhibit uptake OAT transporters. Our preliminary data suggests that these endogenous compounds also impact apical efflux transporters. We hypothesize that 1) both uptake and efflux transport proteins in the proximal tubule are inhibited by uremic solutes (HA, IS, pCS, TMAO) in CKD, and 2) inhibition of transporters by endogenous uremic solutes constitutes the principal cause of the complex nonlinear relationship between renal drug clearance and CLcr, and leads to intracellular accumulation of potential nephrotoxins. These hypotheses will be investigated using PEN and TEN as representative tubular drug transport substrates exhibiting a greater decline of renal drug clearance in CKD than that predicted by estimated CLcr. Oseltamivir carboxylate (OST) will serve as a comparative control, whose renal clearance does follow prediction by CLcr. In order to characterize the mechanisms by which uremic solutes alter tubular transporter protein function and potentiate tubular toxicity, we will use existing transfected cell technology coupled with an innovative three- dimensional, microphysiological, primary cell culture model that will allow, for the first time, dynamic measurement of transepithelial flux and real-time monitoring intracellular accumulation of model substrates penciclovir, tenofovir, and oseltamivir carboxylate. A concurrent clinical study will evaluate the same drugs in healthy subjects and patients with stage 3 or 4 chronic kidney disease. Successful completion of this innovative research program will provide in-depth insight into mechanisms that regulate tubular clearance function in the disease milieu which will lead to fundamental paradigm change in our clinical approach to managing drug dosing in CKD based upon a combination of filtration, tubular secretion, and uremic biomarkers.